Conventionally, since multiphase clocks allow of accurate adjustment of a phase difference between clocks, and thus are generally used in semiconductor integrated circuits with high-speed transmission lines. The difficulty in using a full-rate single-phase clock for high-speed transmission lines has been increasing along with the increase in the speed of the clocks in recent years, and the role played by the multiphase clocks is important.
A ring oscillator is known as an example of a multiphase oscillator that generates multiphase clocks. The ring oscillator includes a plurality of inverter circuits circularly-connected. The ring oscillator oscillates at a frequency based on the delay time of the clock in each inverter circuit and based on the drive capability of the power that drives the inverter circuits, and the respective inverter circuits output clocks with different phases. As an example of a technique for controlling an oscillation frequency of a ring oscillator, an injection clock can be used. Such an injection clock is input to the ring oscillator, and a state of the output of the inverter circuits is forcibly shifted so as to synchronize the oscillation frequency of the ring oscillator with the frequency of the injection clock.
For example, Japanese Patent Publication No. 2011-61325 discloses a voltage control oscillation circuit that synchronizes the oscillation frequency with the frequency of the injection clock by using an injection locking function. The voltage control oscillator is a ring-oscillator-type voltage control oscillator including a plurality of unit cells that receive differential signals with opposite phases, and the circuit controls a load resistance value in the plurality of unit cells by external voltage to control the amount of delay of the differential signals. In the voltage control oscillator, each of the plurality of unit cells includes a variable load circuit that receives the external voltage through a control terminal, and at least one of the plurality of unit cells includes a switch for causing short circuit between output terminals based on an input signal injected from an input terminal.
However, in a case where such a conventional circuit controls and adjusts the phases of the multiphase clocks to be output, the frequencies of the multiphase clocks are also controlled in accordance with the control and the adjustment. Therefore, the conventional circuit cannot independently adjust the phases and the frequencies. Particularly, even if the conventional circuit controls the phases of the multiphase clocks, the conventional circuit cannot maintain the frequencies of the multiphase clocks constant.
Further, the conventional circuit controls load resistance values of the plurality of unit cells in order to control the phases and the frequencies of the multiphase clocks. Manufacturing variations of the load resistance values affect the control of the phases and the frequencies of the multiphase clocks. Therefore, accurate control is difficult in a semiconductor integrated circuit that operates at a significantly high speed.
An object of the present invention is to provide a clock generator that can independently control phases and frequencies of multiphase clocks.
More specifically, an object of the present invention is to provide a clock generator including a mode that can adjust only phases of multiphase clocks output by a multiphase oscillator.
Further, an object of the present invention is to provide a clock generator that can accurately control phases of multiphase clocks while maintaining frequencies of the multiphase clocks constant.
Moreover, an object of the present invention is to provide a clock generator that can accurately control phases of multiphase clocks without being affected by manufacturing variations of circuit elements.